1. Field of the Invention
The present invention relates to a method of joining steel strips in a steel strip continuous processing line and, in particular, to a strip joining method in which the rear end of an advance strip and the front end of a trailing strip are each cut by means of a shearing machine and then welded at abutting end potions of the strips (by, for example, laser beam welding and the like).
2. Description of the Related Art
Continuous processing is performed in a continuous steel strip processing line in order to improve productivity, stabilize quality, improve the yield of coil ends and the like. In this continuous processing, by welding the rear end of an initially transported coil and the front end of a subsequently passed coil at the entrance of each line, so-called xe2x80x9ccoil jointingxe2x80x9d, coils may be continuously transported without stopping the lines. In this case, when welding high carbon steel material, martensitic stainless steels and the like the weld may harden, making weld cracking likely. Moreover, in a rolling line, when there is a difference in hardness between the weld and the base material post-welding annealing is performed because breaking and other problems are likely to occur in production due to deterioration in the mechanical properties thereof.
Next, a laser beam welding machine having a conventional annealing function will be described with reference to FIGS. 11 to 14. Moreover, an outline of the laser beam welding machine having a conventional annealing function explained here is described in page 6 to page 11 of Laser Heat Processing Research Journal VOL. 4, No. 2 1997, xe2x80x9cUsing Laser Beam Welding in a Continuous Processing Line.xe2x80x9d Furthermore, FIG. 11 is a schematic explanatory diagram of this laser beam welding machine at the time of welding, FIG. 12 is a drawing of the area where the ends of the strips abut viewed from a direction orthogonal to the running direction of the strips at the time of welding, FIG. 13 is an enlarged drawing at the time of annealing of the area where the ends of the strips abut viewed from a direction orthogonal to the running direction of the strips and FIG. 14 is a pattern diagram of an input heat amount for a strip in the laser beam welding machine.
In FIG. 11, reference numeral 1 denotes strips which in the present specification are generically termed an advance strip 1a and a trailing strip 1b. The strips 1 run in the direction of the arrow A and during welding the abutting end portions of the advance strip 1a and trailing strip 1b are clamped by a clamping apparatus 2. As shown in FIGS. 12 and 13, the clamping apparatus 2 is constructed of upper clamp members 2a and lower clamp members 2b each of which clamp a respective advance strip 1a or trailing strip 1b. A shearing machine 3 used for cutting the strips moves to a line center (the center position along the width direction of the strip) until the strips 1a and 1b are clamped. When the strips 1a and 1b are clamped the ends thereof are cut-off by means of the shearing machine 3. The shearing machine 3 is constructed so as to move in the direction. of the arrow B and retreats to the position shown in FIG. 11 after cutting the rear end of the strip 1a and the front. end of the strip 1b. Meanwhile, the ends of the strips 1a and 1b are abutted. Also, a laser beam from a laser oscillator 4 is guided to a processing head 6 via a bent mirror apparatus 5.
A back-bar 8 and a high-frequency induction heating apparatus 9 are mounted to a back-bar unit 7. As shown in FIG. 12, the back-bar unit 7 is held such that during welding the back-bar 8 is positioned beneath the abutting end portions of the strips 1a and 1b, and during annealing, is switched, rotating from the position in FIG. 12 in the direction of the arrow C so that the high-frequency induction heating apparatus 9 is positioned beneath the abutting end potions of the strips 1a and 1b. Also, after annealing is completed and welding is to be performed again, the back-bar unit 7 is rotated in the direction of arrow D in FIG. 13 to return to the position in FIG. 12.
With the laser beam welding machine having a conventional annealing function such as described above, after the strips 1a and 1b are abutted, the processing head is moved in the direction orthogonal to the running direction A of the strips 1 and a laser beam introduced from the laser oscillator 4 is irradiated on the abutting end potions of the strips 1a and 1b to perform laser beam welding. Moreover, during welding, the back-bar unit 7 is moved to the line center and the back-bar 8 supports the bottom surfaces of the strips 1a and 1b. When laser beam welding is completed in this manner, the back-bar unit 7 rotates to set the high-frequency induction heating apparatus 9 at the bottom surfaces of the strips 1a and 1b, and the entire region of the weld is subjected to heating and annealing.
As is shown in FIG. 14, an example of an annealing pattern at the above time, temperature is raised (in FIG. 14, t1 shows the time during which the temperature is raised) after a certain time to from when welding is completed. Next, a predetermined temperature is maintained for a predetermined time t2 (for example, 600xc2x0 C.xc3x9730 seconds) and then cooling is performed by air-cooling.
Accordingly, in the laser beam welding machine having a conventional annealing function, since, in order for annealing to be carried-out, re-heating is performed from a low temperature several tens of seconds after welding is completed, and since the high-frequency induction heating apparatus 9 is moved after welding, a lot of time is required to anneal the weld.
However, since cycle times are strictly limited in steel continuous processing lines, it is important that strips be joined in a short time and such a development has been eagerly awaited.
In light of the above circumstances, an object of the present invention is to provide a method of joining strips in a steel strip continuous processing line whereby high-quality butt welding may be performed in short period of time and in a stable manner
In order to achieve the above objects, according to one aspect of the present invention there is provided a method of joining strips in a steel strip continuous processing line in which a rear end of an advance strip and a front end of a trailing strip are each cut-off by means of a shearing machine and then welded at abutting end portions of the cut strips, the abutting end portions are heated from below to a temperature range where annealing effects are obtained by means of a movable heating apparatus which performs high-frequency induction heating and the laser beam is irradiated from above, at the same speed as that of the movable heating apparatus, to laser weld the abutting end portions, the abutting end portions being in a high;temperature state due to the heating.
Namely, as the result of intensive study of welding methods in steel strip continuous processing lines, the present inventors discovered a method of welding wherein a high-frequency induction heating apparatus is moved at the same speed as the welding speed of the laser beam. That is to say, they discovered a welding method in which a laser beam for laser beam welding is irradiated to abutting end portions of strips being put into a high temperature state for annealing heating. By welding and annealing in this manner, the welding and annealing time may be greatly shortened. Moreover, even though a laser beam welding machine in a steel continuous processing line normally performs welding at several KW of power output, the quantity of heat consumed in annealing requires several tens of KW power output. Accordingly, since the quantity of heat required for annealing is much greater than that required for welding, the annealing effects are not affected even if laser beam welding energy is output after annealing.
According to another aspect of the present invention, arc welding or electron beam welding may be performed instead of the laser beam welding. In this case, it is also possible to greatly reduce welding and annealing time.
According to yet another aspect of the present invention, a smooth surface may be obtained by crushing the weld bead with a swaging roller.
According to still another aspect of the present invention, a filler wire may be added to the laser beam weld to improve metallurgical properties thereof.